42 BACTERIA IN RELATION TO PLANT DISEASES. 
These fluids are only a few of many that may be used. Some of them, e.g., 
potato-broth, require special care in preparation. My own method of making potato- 
broth is to Ea the clean pared potatoes rapidly through a grating machine and 
immediately throw the pulp into the re- 
quired quantity of distilled water (which should 
be twice the weight of the potato). The 
beaker is now put into a water-bath and the 
temperature rapidly raised to 55° C. and kept 
there with frequent stirring foran hour. The 
pulp is now filtered from the fluid and the 
latter is immediately put into the steamer. If 
the steaming is long delayed the broth will be 
dark brown (oxidizing action of the potato- 
enzyme on tannins in the presence of air), 
and if the temperature rises much above 
60° C., before the pulp is removed, some of 
the starch becomes gelatinous and the fluid 
will not filter. 
All media which have boiled away to any 
Fig. 37.* Fig. 38. ¢ considerable extent must, of course, be made 
up to the original volume or weight just prior to final sterilization. 
In these culture-fluids observe the rapidity, density, and ferszstency of the 
clouding ; whether the clouding is simple or turbid from the presence of zoogloez ; 
and finally, whether it is uniform in all parts of the tube. Note the character of 
the rim and pellicle, if any are formed, and how soon they appear ; also the amount, 
color, and general appearance of the precipitate. The amount of the precipitate 
varies greatly with different media. Its quality also varies. Sometimes it consists 
of loose, easily separable particles ; in other cases it is a viscid mass which rises as 
a rope-like unit when the tube is twirled (fig. 38). 
Record the formation of acids, alkalies,{ odors, gas-bubbles, stains, crystals. 
Does the fluid become viscid or ropy? Some organisms bring about this condition 
quickly in a variety of media, e. g., Bacterium pericarditidis (Bacillus pyocyaneus 
pericarditidis), others rarely or never. Precipitates in test-tube cultures vary all the 
way from a scarcely perceptible trace to masses a centimeter or more in depth. Do 
not confound chemical precipitates with bacterial growth. Before inoculation always 
examine media in test-tubes for presence of slight precipitates and for contaminat- 
ing organisms. In cultures of rapidly growing species, at optimum temperatures, 
clouding may occur in less than twenty-four hours; with slow-growing species, and 
*Fic. 37.—Tin box for holding scalpels, forceps, etc., to be sterilized by dry heat. About one- 
fourth actual size. A similar tin box which is very convenient for holding sterile pipettes measures 
2 by 3 by 15 inches. 
tFic. 38.—Twirled culture of the olive-tubercle organism in Uschinsky’s solution, showing 
viscidity of the precipitate in old cultures. 
tBacterial ash is alkaline, and this ash must be carefully washed from the platinum loop in dis- 
tilled water each time before it is used to transfer drops of the culture-fluid to litmus paper. The 
wire must, of course, be re-flamed after washing. 
